When Darwin wrote The Origin of Species, the oldest known fossils were from the Cambrian, named after rocks in Cambria, Wales. (Figure 3-3) But the Cambrian fossil pattern didn't fit Darwin's theory. Instead of starting with one or a few species that diverged gradually over millions of years into families, then orders, then classes, then phyla, the Cambrian starts with the abrupt appearance of many fully-formed phyla and classes of animals. In other words, the highest levels of the biological hierarchy appeared right at the start.Darwin was aware of this, and considered it a major difficulty for his theory. He wrote in The Origin of Species that "if the theory be true, it is indisputable that before the lowest Cambrian stratum was deposited long periods elapsed. . . [in which] the world swarmed with living creatures." Yet he acknowledged that "several of the main divisions of the animal kingdom suddenly appear in the lowest known fossiliferous rocks." Darwin called this a "serious" problem which "at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained."